A Similarity Parameter for Scaling Dynamic Inlet Distortion

Abstract

A similarity parameter suitable for use in both analog and digital analyses of time-dependent inlet pressure distortion data has been developed. This parameter, when used in conjunction with the recently developed Method D distortion methodology, facilitates prediction of full scale instantaneous pressure distortion levels from scale model inlet dynamic data. The similarity parameter was developed by relating the inlet size to the data frequency bandwidth by simple acoustic wave theory. This parameter is shown not only to collapse data from different scale sizes of the same inlet but also to produce similar ratios of dynamic to steady-state distortion levels for inlets of different types and sizes operating at or near engine matched conditions. When operating supercritical the increase in dynamic distortion relative to steady-state distortion is seen to be consistent for the various sizes of inlets at constant values of the similarity parameter. Along with the development of the inlet dynamic distortion similarity parameter, this paper treats the subject of analog filtering and digital averaging of the time-dependent pressure data prior to computation of the Method D parameters. Under certain conditions analog filtering and digital averaging are shown to be equivalent. Furthermore, by use of the similarity parameter, consistent selection of either analog filter bandwidth or digital average time can be made for comparison of trends in inlet dynamic distortion between inlets of different types and sizes.